1. Field of the Invention
The present invention relates to small imaging packages and, more particularly, to the packaging of the semiconductor chips employed in the imaging systems of small imaging packages.
2. Background and Related Art
Imaging systems for hand held small equipment, such as, cell phones for example, typically employ image sensing materials in a CMOS chip arrangement. In such arrangements, the CMOS image sensor creates a picture by detecting signals from photodiodes and MOS transistors formed within unit pixels integrated into an array on a semiconductor chip arrangement.
In this regard, CMOS image sensor chip arrangements have allowed for the integration of logic and an imaging array on the same chip. This has been found beneficial in reducing the number of support chips needed in an imaging system, such as, a digital camera or a cell phone. Packaging requirements often make it difficult to place and wire multiple chips increasing the desire to have a total imaging solution on one chip.
However, recent development efforts have caused the amount of logic required to be placed on the same chip as the image sensor to significantly increase. For example, the amount of logic may now include logic circuits to implement image processing, such as, color correction, white balance, random and fixed pattern noise suppression, image compression, and camera functions, such as, lens motor controls for auto focus and zoom function, among other functions.
The fundamental process for creating a CMOS imaging array is compatible with digital logic, but not optimized for such. To create a high quality imaging array, the process needs to be optimized for low leakage. This typically means lengthy high temperature anneals and low doping levels in the diffusion processes. These processes tend to work against high performance, high density, logic. Thus, either the density and performance of the logic suffer, or the process has to be made much more complex, doubling the number of implants needed to make separate wells, and diffusions for the logic and for the imaging circuits.
There is also the problem of noise. High performance logic often creates noise both in the substrate and on the metal levels. This noise can degrade the image quality. Thus, there are a variety of reasons for using separate chips, one for imaging and one for logic.
Image display systems which have separate chips, one chip for logic and another chip for imaging, are known. For example, U.S. Patent Publication 2004/0095495 A1 describes one such arrangement. When using a two-chip approach to implementing imaging systems, various alternatives are available for allocating functions between the two chips. However, independent of how the functions are allocated between the chips, the arrangement and interconnection of the two chips in a small, rugged package within tight space constraints is critical. Moreover, although high. performance logic chips continue to become smaller even with more functionality, the imaging chip with imaging array cannot be similarly reduced without loss of imaging capability. Thus, typically, the logic chip will be interconnected with a larger imaging chip. The ability to effectively interconnect the two chips to one another to in a chip scale package (CSP) and to interconnect the CSP to the carrier package in which the CSP is arranged, is an important consideration in fabricating, for example, a cell phone camera module in a cell phone package.